I Have Often Heard Of Decommissioned Ships,

or car tyres bound together to make artificial reefs,

 but this is the first time I have seen subway cars used, 

since the early 2000s, decommissioned New York City subway cars have been seeded into the waters around New Jersey, Delaware, Virginia and other eastern states, creating infrastructure for fish habitats along the coastal shelf running from New York to North Carolina,

it has been chronicled many times how new fish habitats have been formed using artificial reefs, in late 2000, the Metro Transit Authority began dumping 1200 old cars into the water; in 2007, another 1600 were added to the list, photographer Stephen Mallon has spent years capturing this deliberate subway-dumping activity in action on the surface and underwater, what a great way to use unwanted trains, buses and planes.